1. Field of the Invention
The present invention relates to an optical apparatus having a power transmission mechanism advantageously arranged for moving optical means such as a photo-taking lens to be used for television shooting or the like.
2. Description of Related Art
An optical apparatus such as a photo-taking lens for television shooting or the like has been arranged to drive predetermined optical means with a driving force for zooming, focusing, adjustment of an iris, insertion and removal of a filter, etc.
In the case of a photo-taking lens adapted for television shooting, for example, a lens unit thereof is generally arranged to be driven by rotating an operation ring which is disposed on the outside of the lens unit in a state of being directly linked to the lens unit. The operation ring is arranged to be operated by the hand of a camera operator or driven by an electric motor responsive to the camera operator, according to shooting conditions. A drive unit which is provided for electrically driving the operation ring is disposed at a part of a lens barrel.
The drive unit includes various switches and a power transmission mechanism which is linked to the operation ring of the lens barrel. When some of the switches is operated by the camera operator, the electric motor is actuated to move the operation ring through the power transmission mechanism. When the lens unit is to be manually moved, the camera operator either rotates the operation ring directly by the hand or operates a lever provided on the operation ring to rotate the operation ring.
The operation ring of the photo-taking lens is thus arranged to be drivable either by hand or by an electric motor. Therefore, the power transmission mechanism included in the drive unit is thus arranged to be operated selectively in one of the two operation modes.
FIG. 6 shows the appearance of the conventional photo-taking lens. The photo-taking lens is composed of a lens body 1, a drive unit 2, a focus ring 3, a zoom ring 4 and an iris ring 5. FIG. 7 shows a television camera on which the above-stated photo-taking lens is mounted.
FIG. 8 shows a power transmission path obtained when a predetermined lens unit is to be driven by an electric motor 10 disposed within the drive unit 2. All elements other than a lens barrel and an operation ring 11 are disposed within the drive unit 2. An intermeshing toothed part 11a is formed on the outside of the operation ring 11 around the circumference thereof. A variable speed gear 12 is provided integrally with the electric motor 10. A sleeve 14 is secured to the output shaft 13 of the variable speed gear 12 with a pin 15. An output gear 16 of the variable speed gear 12 is mounted in such a way as to be rotatable with respect to the sleeve 14. A spring 18 is arranged to push the output gear 16 toward the sleeve 14 through a sliding washer 17.
Under a prescribed load torque, the output gear 16 and the output shaft 13 rotate together. However, when the load torque becomes excessive, the sleeve 14 comes to serve as a torque limiter by slipping relative to the output gear 16 so that a force acting on a tooth face can be mitigated to prevent teeth from being damaged. This torque limiter mechanism gives an advantageous effect by absorbing an impact on the tooth face particularly when the lens unit comes to a stop upon arrival at an end of its action caused by high-speed driving.
An intermediate gear 19 is arranged between the output gear 16 and the operation ring 11 to be rotatable relative to a shaft 20 and to be slidable over the shaft 20, which holds the intermediate gear 19. The intermediate gear 19 has an intermeshing toothed part 19a formed on its outer circumferential side and in mesh with both the output gear 16 and the intermeshing toothed part 11a of the operation ring 11.
With the power transmission mechanism arranged in this manner, when the camera operator operates some of operation switches provided at the drive unit 2, a rotating force generated at the electric motor 10 is transmitted to the operation ring 11 through the output gear 16 of the variable speed gear 12 and the intermediate gear 19. As a result, the lens unit interlinked with the operation ring 11 is driven by the electric motor 10.
FIG. 9 shows an arrangement for manually driving the operation ring 11. In this case, a clutch plate 21 is caused to rotate by a clutch lever (not shown) when the clutch lever is operated by the camera operator. The clutch plate 21 is provided with a change-over pin 22, which is engaging the intermediate gear 19 as shown in FIG. 8. The intermediate gear 19 slides to the left over the shaft 20 accordingly as the clutch lever is operated. As a result, the intermediate gear 19 comes away from its position of intermeshing with the output gear 16. With the power transmission path between the driving motor 10 and the operation ring 11 cut off by operating the clutch lever in this manner, the lens unit can be manually driven either by operating the operation ring 11 directly by hand or by operating a lever which is provided on the operation ring.
For selective switching between the two modes of driving the lens unit, i.e., the electric driving mode and the manual driving mode, the position of the intermediate gear 19 which is located between the output gear 16 and the operation ring 11 to intermesh with both of them is shifted to bring the intermeshing toothed part 19a into an engaging position or into a disengaging position. The intermeshing toothed part 19a is generally formed, like a spur gear, on the outer circumferential side of the intermediate gear 19.
Further, the power transmission member which transmits power by intermeshing of gears and the torque limiter member which prevents toothed faces from being damaged by limiting a transmitting torque in the event of rotation under an overload condition are arranged respectively as discrete members within the power transmission mechanism which links the electric motor 10 with the operation ring 11.
With an optical apparatus having the conventional power transmission mechanism arranged in the above-stated manner, in electrically driving some lens unit included in a photo-taking lens, the intermediate gear 19 which is disposed between the output gear 16 of the electric motor 10 and the operation ring 11 intermeshes with both of two gears, i.e., with the output gear 16 of the motor 10 and the intermeshing toothed part 11a of the operation ring 11.
However, since gears generally have backlash at intermeshing parts, the backlash causes a time lag after the commencement of driving the electric motor 10 and before the operation ring 11 follows the electric motor 10. In cases where the operation ring 11 is instantly driven to rotate by driving the motor 10 at a high speed, the time lag presents no serious problem. However, in the event of a slow zooming, fine lens position adjustment or the like, the time lag before the lens unit actually begins to act after a command for driving the lens unit is issued by the camera operator brings about a serious problem. In such a case, it might be impossible to take pictures as desired by the camera operator.
For driving the lens unit, therefore, it is desired to minimize the backlash of the gear train arranged between the electric motor 10 and the operation ring 11. Further, since the selective switching between the electric driving mode and the manual driving mode is performed by the operation of the clutch lever, much time is required for the selective switching, and operability is not necessarily good. Further, the torque limiter member which limits a torque transmitted in the event of rotation under an overload condition and the power transmission member which transmits power by intermeshing of gears are arranged as discrete members around the lens barrel independently of each other. Therefore, the arrangement increases the number of necessary parts, which occupy a large space within the drive unit and make the structural arrangement of the drive unit complex to hinder reduction in size and cost.
Further, as technical art related to the invention, there are U.S. patent applications Ser. No. 08/745,842 filed Nov. 12, 1996, and Ser. No. 09/040,779 filed Mar. 18, 1998.